It is conventional in the pipeline industry to coat steel pipe on pipelines for corrosion protection purposes. Such coatings were applied on the exterior surface of each section or length of pipe, except for exposed short end portions or stubs of the pipe initially left bare or unprotected so that the pipe end sections could be welded together to form the pipeline as it was being laid. The coating on pre-coated section of the pipe is called the “factory coating” (FC).
One type of parent coating that is often used is a Three Layer Coating, which might be Polypropylene (3LPP or Three Layer PP), or Polyethylene (3LPE or Three Layer PE). Three Layer coating is a complete corrosion protection system composed of three elements (layers) working together. The first layer is a fusion bonded epoxy (FBE) coating. The second layer is a chemically modified PP or PE adhesive. The third layer is a co-polymer outer layer in varying thicknesses, usually several millimeters thick. Combined as a system these elements work together to protect the pipe from undesirable corrosion, wear and impact damage.
After welding the sections together, the uncoated area around the weld must then be coated to create a continuous corrosion protection with the “factory coating”. This area is called the “field joint” or “joint” (FJ).
The joint area also had to be provided with a corrosion protective coating system. A compatible field joint coating system for 3LPP or 3LPE coated pipelines comprising a heat assisted helically wrapped tape application system from U.S. Pat. No. 7,243,697, is known prior art. This system utilizes existing equipment for abrasive blasting or surface preparation and application of a first fusion bonded epoxy (FBE) layer, then covered with chemically modified polypropylene (CMPP) or polyethylene (CMPE) dual powder second layer. Then a third layer is immediately applied by the machine helically wrapping applied polypropylene (PP) or polyethylene (PE) tape, where the tape overlaps each preceding wrap and is also overlapping the factory coating to create a continuous coating. A known drawback of this solution arises from the multitude of overlapping jointed layers.
Another joint coating is provided by longitudinal sheet wrap which involves wrapping a sheet of thermoplastic material around the pipe. From U.S. Pat. No. 6,440,245, it is known a ready-to-use multi-layer coextruded sheathing comprising at least the 3 layers system in a single sheathing. The multi-layer sheathing is previously heated and then transferred with a roller having a larger diameter than the pipe, around the pipe. According to U.S. Pat. No. 6,440,245, the applicator roller needs to be at least the pipe diameter or bigger. This feature restrict the ability of the machine to coat large diameters pipes, when line pipes are already landed and welded over kilometers, because, it will be no more possible to change the line pipe height relative to the ground.
Now, it is known from US-2014-0076483, a machine for coating a pipe joint with a cylindrically wound unheated sheet of synthetic resin on the joint that is already coated with 2 layers. The machine includes a mounting frame mountable on adjacent coated sections of the pipeline to span the pipe joint being coated and a transport carriage rotatably movable on the mounting frame to move circumferentially about the pipe joint. A storage reel is mounted on the transport carriage for storing a supply of the unheated sheet of synthetic resin to be wound about the pipe joint. An applicator roller is mounted on the carriage frame and applies the unheated sheet of synthetic resin from the storage reel onto the pipe joint as the transport carriage moves about the circumference of the pipe joint. A heater is mounted on the transport carriage to heat the applied unheated sheet of synthetic resin after it has been applied to the pipe joint, to preheat the substrate and the factory coating overlap area, the newly applied unheated sheet of synthetic resin material, and the previously applied material on subsequent revolutions of the applicator.
This machine has several drawbacks because it needs to develop a roller with specific heating capabilities for each type of synthetic resin sheet. This system needs to apply the sheet just after having coated the first two layer of the 3LPP or 3LPE system. This feature is not allowing intermediate control of the first two layers, and in rendering temperature control very difficult due to inevitable change in the production line speed. And the main drawback is the risk of having air trapped between each successive layers of wrapped sheet material provided for an efficient protective coating.
It is also known from WO-2008/107759 a method of forming a protective joint coating around a weld, the method comprising the steps of: laying a thermoplastic sheet on a supporting sheet, winding the thermoplastic sheet and the supporting sheet about the weld to join the thermoplastic sheet to the weld, the supporting sheet being removed when the protective coat is completed. This solution is unsatisfactory, as it requires to add a cement layer of Gunite to bridge the thermoplastic sheet with the factory coating. Moreover the management of the supporting sheet according to this method is difficult to perform without damaging the thermoplastic sheet, because it requires to move a roller in such a way to detach the supporting sheet from the thermoplastic sheet.